1. Field of the Invention
This invention relates to the x-ray lithography used for the production of fine patterns on semiconductor substrates or wafers to produce very large scale integrated circuits. More particularly, the invention concerns a step and repeat alignment apparatus and method of linear zone plate aligning of pattern masks and semiconductor wafers writing monochromatic laser illumination.
2. Prior Art
Alignment by the use of linear zone plates is described in U.S. Pat. No. 4,311,389 Bernard Pay et al. A single laser source of monochromatic light is employed to illuminate a linear Fresnel zone plate (LZP) formed on a mask which light passes through transparent areas on the linear Fresnel zone and reflected from a reflecting grating inscribed on a wafer and a return beam diffracted by the Fresnel zone marks. While this apparatus and method has been successfully used, a problem has existed due to the extreme monochromatic nature of the laser illumination and the fact that typical wafers have many thin layers of silicon oxide, silicon nitride, polysilicon or the like thereon which interact with the laser beam to improperly diffract light to the wafer targets. This may also be caused by the particular thickness(es) of the thin layers. As a result, the alignment signal will be too low to obtain an accurate alignment. While it is possible to adjust the integrated circuit manufacturing process to alter the thickness of the layers on the wafer, in many cases, other constraints, e.g., electrical properties of these layers prevent a user from doing so.
In order that light diffract from a grating, there must be a periodic difference in phase or transmission in the grating. The grating of concern is the wafer target used for the LZP alignment system. If the layer on the wafer which forms both the target and the background absorbs the alignment illumination to the same degree, and if the phase retardation of the two areas on the wafer is the same, then diffraction of the light will not occur. Actually, other combinations of phase retardation and absorptions will also give null diffraction, i.e., they will cancel each other out. Small perturbations from the null cases will give some signal, but the signal must be above a certain level before accurate alignments can be made.